Why is being a prime minister of Japan such a short-lived affair? There have been six of them in five years. It used to be said that after more than 50 years of almost uninterrupted rule, the Liberal Democratic party had run out of energy. Junichiro Koizumi was the last man who could credibly fill the boots of a leader. But the same is now also true of the ruling Democratic party of Japan, which appears to be discarding its leading lights with similar abandon. Ever since the DPJ ended the LDP's virtual monopoly of postwar power two years ago, successive governments have been in a state of crisis.

The first DPJ prime minister, Yukio Hatoyama, promised more than he could deliver, particularly on a pledge to relocate US marines and one base out of Okinawa. His successor, Naoto Kan, has repaired the relationship with Washington, but only at the cost of distancing his government still further from the wishes of most Okinawans. Both the left and, cynically, the right on the island are against such a heavy US military footprint. Mr Kan has toiled instead on two fronts – breaking the gridlock in parliament over the budget, and cleaning up a series of funding scandals that have dogged the party since its inception. He suspended the membership of a party grandee and founder, Ichiro Ozawa, after he had been indicted in a political financing case. Mr Ozawa cast a long shadow over the government, taking 16 DPJ members of parliament with him. A key warrior in Mr Kan's fight with Mr Ozawa was his feisty foreign minister, Seiji Maehara. Now he himself has had to resign after receiving an illegal donation, and Mr Kan's chances of survival are looking slim indeed.

Mr Maehara's resignation is a loss. He had both the vision and energy to navigate a military alliance with the US and an increasingly important trading relationship with China. Having been one of the leading critics of Mr Ozawa, he felt compelled to resign, even though the amounts involved in his case were trivial. But it is a measure of Mr Kan's political fragility that even bright prospects for the future can not survive what any other democracy would treat as minor technical infringements. An expenses scandal this is not.

There is no consensus on why Japan's prime ministers are so frail. Some put it down to the fact that the ultimate stability resides in the emperor, and dismissing a prime minister is no more traumatic than changing the coach of a baseball team. Others point to the postwar education system. Prime ministers have relatively low salaries, little authority even within cabinet, and limited terms. Whatever it is, Japan could do with one who can stay around for more than a year. All applications are welcome.